Regardless of whether it was a marshall (Bernadotte again, yes) or even one of the brothers (Joseph or possibly Louis), I feel like France, Austria, Prussia and Russia would come to an arrangement that would 1) bring peace to the continent and 2) frustrate Britain. IOTL, Britain had initially considered allowing one of the brothers or even Napoleon II thru the regency of Marie-Louise to remain in power before emigre agitation and British public opinion forced them to demand a restoration of the Bourbons. Metternich was frustrated by British intransigence and feared the military might of Russia so he ultimately caved in to Britain's demands in order to prevent Europe from exchanging one form of dominance for the other.
If the next Emperor of France were to quickly take advantage of the lull in fighting and reach accomodations with the Eastern powers, France would only have Britain to deal with, and Britain would likely be forced by mercantile interests and lack of continental allies, to sue for peace as well. The tough part would be in discussing terms in this alternate Franco-British peace treaty, for I feel that the negotiations with the Eastern powers would prove to be somewhat easier because they wouldn't concern themselves with British interests. How this peace treaty between France and the Eastern powers may be something like this:
1) France would retain the territory on the left bank of the Rhine (bringing them their 'natural limits') in exchange for evacuation of Italy and the rest of Germany
2) Austria would be conpensated with the acquisition of Lombardy-Venetia (as IOTL), and tiny pieces of Bavaria.
3) Prussia could gain nearly all of Saxony and some territory on the right bank of the Rhine to serve as a buffer between them and France. They would likely regain some lands from the Grand Duchy of Warsaw as well
4) Russia would gain the lion's share of the former Grand Duchy of Warsaw and Finland.
5) I could even forsee a situation in which a German Confederation similar to the one IOTL would be created, but jointly presided by Prussia and Austria and guaranteed by Sweden and Russia.
I like the general idea of the universal peace under Emperor Joseph (unfortunately, no chance for Bernadotte even if he is my personal favorite but don’t you worry, he would be very close to the throne 😂).
But I have few comments/objections:
1. At that time France is at peace with Austria (and Austria is beaten seriously enough not to be a major player for a couple years).
2. We are talking about 1807 so with the exception of the Konigsberg area Prussia is occupied and most of its army surrendered. Getting the lost territories back would be a huge gain for Prussia.
3. While the Poles successfully liberated a big part of the Prussia-held territory, the Duchy of Warsaw (without “Grand” 😜) was
officially created only at Tilsit.
4. In an absence of the CS Britain is not “isolated” because its trade with the German states, Russia and Sweden continues. OTOH, it’s absence creates a potential for the peaceful negotiations.
5. France is not defeated militarily and, with Russia and Prussia exhausted, there is no reason for the 4th Coalition to be too cocky.
Based upon the above, I don’t think that your ....er... generous territorial concessions (😜) would be on the table:
1. Prussia would be lucky to get back its pre-war territory including a “right” to re-conquer Prussian Poland from the Poles (throwing them under the bus may smell badly but
formally France did not have any obligations). Saxony is definitely off the table as a member of the anti-Napoleonic 4th Coalition until December of 1807.
2. Russia is not getting any territories and at that time did not have any claims to the territorial acquisitions: 3rd (with the exception of Corfu) and 4th coalitions were absolutely altruistic exercises of idiocy caused by Alexander’s personal feelings toward Nappy. Sweden is Russian ally in the 4th Coalition so no Finland.
3. Germany is tricky. France definitely gets border on the Rhine but may retain some footprint to the East of it. This is an issue which can bring Britain into the discussion: by 1807 France is controlling pretty much all German coast and willingness to negotiate and keep the German markets open may be important enough for the Brits to compromise on the subjects of Belgium and Holland. Confederation of the Rhine may survive.
4. Austria, as a neutral state, is not a part of a deal but, if need arises, it may get back some of the territories lost in Italy. In general, fate of the Italian states may be a subject of the separate negotiations between France and Austria (perhaps with the Brits chipping in as well on the subject of Naples).
